Comprehensive analysis and design of a quasi two-level converter leg

Quasi two-level operation of voltage source converters, intended to operate either within high/medium voltage DC networks or low output frequency applications, has caught the attention of both academia and industry. Sequential insertion of the so-called submodules (cells) into the circuit alleviates potential problems related to high voltage slopes (dv/dt), whereas the ease of stacking them together provides the means for satisfactory performance in terms of voltage scalability. Although resembling the modular multilevel converter, quasi two-level converter operates in a different manner, leading to the completely different sizing, operating and balancing principles. So far, no publication has analyzed the transition process occurring within the quasi two-level converter leg, which can be considered crucial from the cell capacitance sizing perspective. Therefore, this paper provides thorough and generalized mathematical description of converter leg dynamics during the voltage transition process. Comprehensive method giving the possibility for cell capacitance determination is presented. With the aim of validating the conducted analysis, simulation results covering the operation of a few different topologies, with varying number of cells, are presented.